The present invention relates generally to television signal discrimination systems in television receivers and more particularly to a system which, in the automatic searching of channels in a television receiver having an electronic tuner of memory type, accurately discriminates only the regular or normal television signal without storing by error a datum of a spurious television signal.
In general, as a television receiver of electronic tuning-channel selection memory type, a television receiver provided with an electronic tuner, a memory in which data relating to channels whose signals are to be received have been stored, and a microprocessor for operating these components has been known. When a desired channel is to be selected, the microprocessor causes the electronic tuner to undergo channel selection in response to the stored content of the memory.
When a television receiver of the above described system is being installed in a certain reception place, it is necessary to cause the memory to store the data relating to the channels which can be received at that reception place. As a system for automatically searching for all channels which the receiver can receive (hereinafter referred to as automatic channel search) and causing the memory to store data in accordance with the frequencies of all of the channels, a so-called voltage synthesizer system is known.
In this system, as will be described hereinafter in conjunction with an accompanying drawing, an electronic tuner undergoes automatic channel search operation in response to an indication from a microprocessor, which detects the reception of the signal of a certain channel from the output of a frequency discriminator and the vertical synchronizing signal separated from the received television signal and causes the memory to store data conforming to the frequency of that channel.
However, in the case where the television receiver is installed under a strong electric field, a higher harmonic wave is generated by a high-frequency amplifier in the electronic tuner. Furthermore, cross modulation and intermodulation occur between the television signals of two or more channels. In this case, a spurious television signal differing from the television signal of the normal channel is generated as a useless signal in a frequency position other than the normal channel frequency over a wide frequency range. In a spurious television signal of this character, in addition to a vertical synchronizing signal in every vertical scanning period, there exist one or a plurality of other vertical synchronizing signals within that period. Furthermore, on the output detected video signal waveform, the entire television signal level is subjected to modulation, whereby the envelope is fluctuating.
Since this spurious television signal is originally a useless signal and should not be channel selected, it should not be stored in the memory during automatic channel search. However, in the known automatic channel search system, erroneous operation due to the vertical synchronizing signals of the spurious television signal occurs, and the data of this spurious television signal also becomes disadvantageously stored.
Accordingly, a method of detecting the presence or absence of a useless vertical synchronizing signal within one vertical scanning period would appear to be possible. However, if the microprocessor were to have a circuit organization for continually carrying out the above mentioned detection, it would become expensive and impractical. Furthermore, it would be difficult to distinguish between a useless vertical synchronizing signal and a simple external noise.